Body processes

Passive mass transport – function, task and diseases

Passive mass transport

Passive mass transport is the diffusion of substrates through a biomembrane. This diffusion takes place along the concentration gradient and requires no energy. The diffusion process can, for example, be disrupted in the intestines of HIV patients.

What is passive mass transport?

Cells or cell formations are separated from each other in the body by a biomembrane. Due to its specialized structures, this flexible separating layer enables the transport of specific molecules and information into and out of the cell interior.

There are two basic modes of transport of substances in and out of the membrane. Membranes have selective permeability . They allow some substances to diffuse while being a barrier to others.

Due to the active transport mode of mass transport , membranes can open up in a targeted manner for molecules for which they are actually not permeable due to their charge, their concentration or their size. Active transport always takes place with the expenditure of energy. This must be distinguished from the type of mass transport of passive transport. No energy is required for this form of material movement through a cell membrane . Passive transport can be equated with diffusion processes that take place along the concentration gradient and produce a concentration equalization between the two sides of the membrane.

function & task

In a cell or a cell compartment, there is a certain chemical and charge environment that is required for the cell to function properly. This milieu is maintained solely by the properties of the biomembrane and selective permeability. The passive and active transport of substances supply the cell or the cell compartment with exactly the right amount of substances that are required for a beneficial environment. 

There are two different types of passive transport. Simple diffusion affects lipid-soluble molecules and occurs at an extremely slow rate. They diffuse freely through the cell membrane. This form of passive transport is the one with the least effort. The second type of passive diffusion is facilitated diffusion, which in turn can be divided into two sub-types. One of these sub-forms is carrier-mediated facilitated diffusion. In this form of passive mass transport, the membrane absorbs the substrate with the help of a so-called carrier. The carrier is a proteinto identify the substance to which the substrate binds. Because simple diffusion occurs at low speeds, the carrier helps transport the substance across the biomembrane. The number of all carrier molecules is limited.

For this reason, the transport through a carrier molecule is subject to saturation kinetics. The passive mass transport by carrier molecules can also be subject to competitive inhibition . When a carrier molecule binds to its substrate, it changes conformation and rearranges itself accordingly. This transports the substrate molecule through the biomembrane and only releases it again on the opposite side.

Some carriers can only transport one molecule at a time and thus have a uniport. Other carriers have binding sites for two different molecular substrates and only change the conformation when both binding sites are occupied. Thus, the two molecules are carried either symport in the same direction or antiport in opposite directions. There is no dependency on the electrical gradient.

The second type of facilitated diffusion is through pores and channels. This form of transport particularly affects amino acids . For example, the substrate of the amino acid is taken up through pores in the cell membrane during ion transport. The channels are formed by proteins. Special binding sites are present on these protein-containing channels. Thus, the facilitated diffusion through pores and channels is a selective mass transport that can be influenced electrically and chemically.

Almost all channels are only opened on certain signals. A ligand-controlled channel, for example, only reacts to a messenger substance such as a hormone . Some channels are voltage gated and open to diffusion with a change in membrane potential . After the concentration equalization, the channels close again.

Diseases & Ailments

If the membrane permeability and thus also the passive transport of substances is disturbed, the permeability of different ions is no longer ideally regulated. Such membrane permeability disorders often develop from cardiovascular diseases and sometimes impair the electrolyte balance. 

Sometimes disorders of membrane permeability are also hereditary. Various proteins build up the biomembrane and give it a selectively permeable double lipid layer. If the proteins involved are changed, the membrane permeability also changes. This phenomenon is present, for example, in the myotonia congenita Thomsen . This genetic disorder of muscle function mutates a gene that codes for each chloride channel in muscle fiber membranes. Due to the mutation, the permeability to chloride ions is reduced, causing muscle stiffness.

Autoimmune diseases can also target the biomembrane, such as antiphospholipid syndrome . During the disease, the immune system attacks the phospholipid-bound proteins in the membrane. At the same time, the increased blood clotting tendency increases the risk of heart attacks and strokes .

Mitochondriopathies also change the permeability of the membranes. The mitochondria are the body’s own energy power plants, which shed free radicals when generating energy . In healthy people, these substances are intercepted. This process fails in patients with mitochondrial disease, damaging the membranes and greatly reducing the mitochondria’s energy-producing ability.

The passive and active transport of substances through the membranes of the small intestine is particularly affected by disorders in diseases such as HIV enteropathy. This phenomenon primarily affects HIV patients with chronic diarrhea and may be associated with reduced activity of interstinal enzymes.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.